The estrogen receptor (ER) is a transcription factor that mediates the expression of estrogen-activated genes. The ER has been associated with a variety of diseases including breast cancer, osteoporosis and cardiovascular disease, and is therefore an important target for therapeutic intervention.[1] Binding of an estrogen molecule to the ligand binding domain (LBD) of the ER ultimately leads to interaction with specific DNA promoters and recruitment of coactivactor proteins. These coactivator proteins mediate the assembly of the transcriptional machinery and are therefore essential for expression of the ER-regulated genes. Traditionally, inhibition of the ER has been attempted by using antagonist molecules that bind to the LBD and trigger a conformational change that prevents the ER from recruiting the coactivator proteins.[2] An alternative and underexploited approach involves the small molecule inhibition of the interaction between the estrogen-activated ER and the coactivator proteins.[3a,b] Importantly, it has been shown that an analogous strategy can be used to target other nuclear receptors.[3c]
The coactivator proteins possess multiple copies of a conserved LXXLL motif also known as nuclear receptor box (where L is leucine and X is any amino acid). Extensive studies have shown that this short LXXLL sequence is important and sufficient for binding to the ER.[5] The X-ray structure of the ligand-bound ER and a fragment of the coactivator GRIP1 shows that the LXXLL peptide adopts an α-helical conformation where the leucine side chains in positions i and i+4 are projected into a hydrophobic groove on the ER surface while that in the i+3 position projects into a hydrophobic pocket.[4] Additionally, the crystal structure suggests that interactions between the coactivator peptide backbone and the charged residues that flank the binding groove on the ER further stabilize the complex.
In the search for inhibitors of this interaction, various short peptide derivatives based on the LXXLL sequence have been shown to disrupt the ER/coactivator interaction.[6] However, there have been only two reports of small molecule inhibitors with only one of them (with a Ki value of 29 μM) designed to bind to this surface region of the ER and block the coactivator's approach.[3]
We have previously reported a broad strategy to the disruption of α-helix/protein interactions that involves the design of rigid scaffolds from which groups mimicking the surface functionality of an α-helix can be projected.[7] For example, 2,3′,3″-trisubstituted terphenyls can mimic the i, i+4, and i+7 residues of two turns of an α-helix and lead to potent inhibitors of protein/helix contacts such as those between Bcl-xL/Bak and MDM2/p53.[8] In the case of the coactivator LXXLL motif, a modified approach is needed to incorporate the features of the i+3 leucine. We and others have shown that this can be simply achieved by placing a second ortho-substituent on a biaryl scaffold.[9] Separation of the elements of the i+3 side chain by a single methylene allows the adoption of a relative side chain conformation on the biaryl that closely mimics the distances and angular projections of the i, i+3, and i+4 groups of an α-helix.